Steel in the News
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Posted by Ted Sheppard on June 14, 2011 at 2:17 PM.
Jacks come in all shapes and sizes. They are versatile tools. If you want to make small, very sensitive movements, there are jacks for that. If you want to lift 1,000 tons, there are jacks for that. From my perspective, jacks have evolved over time from my first field experience with jacks to my latest experience.
On my first bridge project we had screw jacks that would move wedges in and out to lift or lower what they were supporting. To assist in the sliding the wedges were usually greased. This meant that you too were greased after you handled them. Another problem with them was that you were on top of a falsework bent, and there wasn’t always enough room to get out to the end of the long handled crank that turned the screws. You could reach it, but you could not always exert enough power to move the wedges. On my first bridge this occurred. The bridge and a roadway were almost parallel, so we rigged a manila line down the falsework, over to a block attached to a tree on the river bank, and then fastened it to the trailer hitch on our foreman’s station wagon. He ran up and down the road a few times, and we got the bridge jacked. Fortunately it was not a large adjustment or we would have paid the foreman mileage.
I soon graduated to hydraulic jacks. These were much more reasonable, but not without their own problems. My first hydraulic jacks were water jacks with hand pumps. Yes, water jacks, and yes, in winter we added anti-freeze. And yes, even then they froze up. On my first water jack job, we had 500 ton capacity jacks. They were big and heavy. During the temporary closure of the arch ribs on the Niagara Arch Bridge, one of the jacks on my side of the river froze. We had to wrestle it out from under the falsework and put in our spare. A very big engineer from the home office, who loved being in the field, was handling the jacking on the other side of the river. He kept chirping into the radio, “I got mine,” or “This side is ready.” It was very cold, but I was steamed. We finally made it, and I didn’t lose my job. Patience really is a virtue.
I moved up to jacks that used hydraulic fluid with electrically driven pumps. We had to jack a 1,400 ton space frame that had been assembled on the ground 85 ft in the air. We used four jacking towers that swallowed the four bearing points. The concrete pylons were built inside our towers, and we then lowered the frame to its final position. Our pumps on this job could be adjusted so that the speeds were equalized at all four towers.
When I was at Tri State Steel Construction we had to raise the elevation of a bridge with a minimum delay to traffic. We used an electrical console to control 40 jacking points, and we raised each point simultaneously. We would go one full stroke, add temporary supports, “fleet” the jacks (see note below), and after the general contractor had placed a wedge of asphalt, we would go again. The temporary supports actually remained in the concrete of the new bearing pedestal.
I then moved on to strand jacks. When we were awarded the bridge over the Kanawha River in West Virginia, we had planned on using falsework in the river. Although this would have been permitted per the specifications, the U.S. Coast Guard asked if we could erect the bridge without falsework so that there would be no impediment to barge traffic. We looked at it and decided to erect the center span of the twin tub girder spans by using strand jacks. The center span was 450 ft long and the assembled girders weighed about 400 tons. We put a strand jack on each end of the side spans, assembled the girders on barges, floated them into position and jacked them into place.
You can see that large or small movements or lifts that weigh a few tons and as many as thousands of tons can be handled efficiently with jacks. The jacking systems today, just 10 years since my last retirement, can do marvelous things. You won’t get greasy, and the jacks won’t freeze up. When you are planning a job, if it is appropriate, consider using jacks.
Editor’s Note: We asked Ted to explain what it means to “fleet” the jacks for those who are not familiar with the term.
When a single, full-length stroke of the piston, does not give you the total lift distance required, you must fleet the jacks. This means, putting a temporary support under the piece being jacked, removing the jacking fluid to retract the piston, adding a shim or pipe piece to make up the difference for the movement of the first stroke, and then jacking again. This happens a lot on bridges where the space available is much less than that needed to jack up with one stroke. Some jobs require several cycles of jack, shim, retract, shim, jack, etc. If all the jacks worked exactly the same and all of the shims could be placed in the same time frame, you could do this to music. On the project described in this article, we did this at night. I wasn’t interested in singing; just getting to the hotel and climbing into bed.